1. Field of the Invention
This invention relates to an optical disc apparatus for recording/reproducing and, more particularly, but not exclusively, to optical disc apparatus of the write once read many times type.
2. Description of the Prior Art
It is known to provide so-called write once, read many times optical discs so that desired information may be recorded and read. In at least some write once, read many times optical disc devices, a light beam is caused to illuminate an organic pigment type thin film to form pits whereby desired information may be recorded on a once only basis. By forming pits in this manner, reproduction by an ordinary compact disc player of this type of optical disc is possible. Accordingly, this type of optical disc may be conveniently used in cases where only a small quantity of discs capable of being reproduced by a compact disc player need be produced without having to prepare a stamper for each disc.
In this type of optical disc, a pregroove is formed at the pit forming position and time information (ATIP: absolute time in pregroove) is recorded in the pregroove. Thereby, in an optical disc device manipulating such a disc, desired information may be recorded/reproduced on the basis of such time information.
In addition, in this type of optical disc, a carrier signal at a frequency of 22.05 kHz is frequency-modulated by address data and the pregroove is wobbled (moved radially) by the resulting frequency-modulated signal. In the optical disc device, the reflected light from the optical disc is received at a light receiving element of which the light receiving surface is divided into two portions in the radial direction of the optical disc. By detecting the wobbling of the pregroove based on such received light, a spindle motor may be rotated to hold the reproduced carrier signal at the frequency of 22.05 kHz and thereby rotate the optical disc at a predetermined rotational speed. Furthermore, the optical disc device is adapted to detect the time information by demodulating the output signal of the light receiving element to yield address data.
In the output signal of the light receiving element, time information consisting of address data and speed information for driving the spindle motor may be securely detected before the forming of the pits which carry the recorded write once read many times information. However, after forming the pits, leakage of a signal component of the recorded information (hereinafter referred to as the RF component) into the output signal cannot be avoided due to the fact that the incident light upon the light receiving element is modulated by the pits. In addition, the frequency band of the RF component and that of speed and time information partially overlap as shown in FIG. 4 of the accompanying drawings.
Conventionally, in the optical disc device, a difference signal is generated from the output signals of the light receiving surfaces of the light receiving element, the light receiving surface of which is divided into the two portions to detect meandering of the groove. When the signal levels of the RF components are equal to each other in the two output signals, it is possible for them to cancel each other's RF component in the difference signal. In this case, speed information and time information may be securely detected.
However, the output signal levels of the light receiving surfaces may be different from one another in the optical disc device. In this case, the RF component cannot be completely removed due to the fact that the signal level of the RF component is different between the output signals, and therefore speed information and time information cannot be detected.
One possibility for solving this problem, would be that in which a variable resistance, for example, is adjusted to adjust the two output signal levels. However, the incident position of the light beam with respect to the light receiving element in an optical disc device may change due to temperature change and/or aged deterioration. Accordingly, in this case, though speed information and time information may be securely detected directly after such adjustment, it will eventually become impossible to detect speed information and time information. Further, when the signal level of the RF component is changed between the output signals due to tracking errors during reproduction, removing of the RF component is likewise impossible.
U.S. Pat. No. 4,942,565 discloses an optical disc apparatus of the type using wobbling of the track to store control information for the operation of the apparatus.
U.S. Pat. No. 5,185,732 discloses an optical disc device in which track width is used to store information.
It is an object of the present invention to provide an optical disc device in which speed information and time information recorded in the pregroove may be securely detected even in such cases where the incident position of the light beam with respect to the light receiving element changes.